Rigid probe with compliant characteristics

ABSTRACT

A method of probing compliant bumps of a circuit with probes is described. The method includes disposing the probes on a substrate, a base of each of the probes being coupled to the substrate. The method also includes disposing the circuit such that each of the compliant bumps is in contact with the probe tip of a corresponding one of the probes, each probe tip being connected to each base of each probe through a cantilever, and supplying current to the probes to test the circuit.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.13/720,092 filed Dec. 19, 2012, the disclosure of which is incorporatedby reference herein in its entirety.

BACKGROUND

The present invention relates to wafer probing, and more specifically,to fine pitch probing of high power integrated circuits with compliantbumps.

Controlled collapse chip connection (C4) is a method that interconnectssemiconductor devices such as integrated circuits of a wafer with solderbumps. Semiconductor devices are typically tested in wafer form beforedicing into individual chips for further packaging, test, and sale. Forhigh powered devices, hundreds or thousands of solder bumps or pads onthe chip are brought into contact with a wafer probe system attached toa test system. The wafer probe system has corresponding probe tips thatmake electrical connections to respective pads or solder bumps.

Two types of probe systems have been in use, compliant and rigidsystems. Compliant probes are designed as springs, often in the form ofa cantilever, buckling beam, or coil spring. The spring has therequisite force to break thru surface oxides and initiate electricalcontact with a compliant bump or other pad, but needs to be relativelylong in order to achieve the desired compliance to overcomenon-planarities between the bumps and probes (i.e., the fact that allthe bumps are not equidistant from corresponding probes). Thus, thistype of probe is limited by the proximity of pads for which it can beused. Compliant probes also tend to be relatively expensive for largearrays, and the longer length results in poor electrical performance(inductance and resistance) and overheating at high currents.

Rigid probes, such as a Thin Film Interposer (TFI) probe presented Jun.6 2007 at Southwest test Conference, are much shorter and relativelyless expensive than compliant probes, but rely on the compliance(deformation) of the bumps to overcome non-planarities and make fullcontact to large arrays. Although the test chip would ideally bestationary during testing to prevent a loss of connection with theprobe, various testing elements cause thermal and mechanical stressesduring test that result in unwanted movement of the chip under test orprobes. For example, during the various tests, the probe forces currentthrough the solder bumps. The current flow causes temperature changes.This temperature change and resultant dynamics are generally morepronounced for higher power chips. This movement can cause loss ofelectrical contact when rigid probes are used to test plasticallycompliant bumps such as solder bumps.

SUMMARY

According to another embodiment of the invention, a method of probingcompliant bumps of a circuit with probes includes disposing the probeson a substrate, a base of each of the probes being coupled to asubstrate; disposing the circuit such that each of the compliant bumpsis in contact with the probe tip of a corresponding one of the probes,each probe tip being connected to each base of each probe through acantilever; and supplying current to the probes to test the circuit.

Additional features and advantages are realized through the techniquesof the present invention. Other embodiments and aspects of the inventionare described in detail herein and are considered a part of the claimedinvention. For a better understanding of the invention with theadvantages and the features, refer to the description and to thedrawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The forgoing and other features, and advantages ofthe invention are apparent from the following detailed description takenin conjunction with the accompanying drawings in which:

FIG. 1 depicts an array of probes according to an embodiment of theinvention;

FIG. 2 is a cross-sectional view of a wafer test system according to anembodiment of the invention;

FIG. 3 is a perspective side view of a probe according to an embodimentof the invention;

FIG. 4 is a perspective side view of a probe according to an embodimentof the invention;

FIG. 5 illustrates the deflection of the probes according to anembodiment of the invention; and

FIG. 6 is a flow diagram of an exemplary method of probing compressiblebumped structures according to an embodiment of the invention.

DETAILED DESCRIPTION

As noted above, wafer probing, especially of high power chips withcompliant bumps tested with rigid probes, is susceptible to loss ofconnection between the chip and the probe due to insufficientelastically compliant characteristics of the probe and bump. Further,wafer probing with compliant probes limits the pitch (compliant bumpdensity) that can be tested. Embodiments of the invention describedherein disclose a probe and a method of probing that address the need tomaintain electrical contact between the probe and the solder bump duringwafer testing. While C4 interconnects with solder bumps are discussedspecifically herein, the probe and probing method discussed in theembodiments below apply to any compressible bumped or column structures.As detailed herein, embodiments of the probe supply sufficient force todeform the solder bumps and initiate electrical contact, yet includecompliant characteristics such that the probe follows any movement ofthe chip to maintain contact during testing. Embodiments of the probeare also compact enough to fit between adjacent C4s and are sufficientlycost effective for chips with a large number of contacts.

FIG. 1 depicts an array of probes 100 according to an embodiment of theinvention. The probes 100 are shown on a substrate 120 with viaconnections 130. The substrate may be silicon, ceramic, organic, oranother suitable material. The via connections 130 may carry currentsupply to the probes 100 to be applied to the solder bumps 220 (FIG. 2).Each probe 100 includes a base 110, a probe tip 112, and a cantilever115 that connects the base 110 and the probe tip 112. As shown in FIG.1, an exemplary probe tip 112 and an exemplary base 110 may becylindrical in shape. The surface of the probe tip 112 that makescontact with the solder bump 220 may have patterns (not shown) builtinto the probe tip 112. The pattern can help to disturb any oxide orcontamination on the solder bump 220 to initiate the contact. Thepattern may be, for example, pyramidal or smaller cylindrical patternatop a relatively larger cylindrical pattern. The material forming theprobe tip 112 affects its size and shape. That is, a probe tip 112 madeof a relatively strong material can be relatively smaller than a probetip 112 made of a relatively weaker material and still apply thenecessary force to the solder bump 220 to initiate contact. In addition,the material forming the probe tip 112 may be based on the materialforming the compliant bump (e.g., solder bump 220). For example, if thesolder bump 220 is made of tin, then a probe tip 112 made of copper (ofa sufficient size and shape) will deform the solder bump 220 to makecontact. However, a tin probe tip 112 would not sufficiently deform acopper compliant bump, for example.

The cantilever 115 between the base 110 and the probe tip 112 may have arectangular shape or may have a curved shape to accommodate adjacentprobes 100. The distance between the probes 100 must correspond with thedistance between solder bumps 220, which is referred to as the pitch(P). Because different chips under test 210 (FIG. 2) or even differentsolder bumps 220 on the same chip have different distances between thesolder bumps 220, the pitch (P) is one of the parameters that may becustomized for compatibility with a given chip under test 210. A typicalpitch may be, for example, between 25 and 200 micrometers (μm). However,as shown by FIG. 2, for example, the cantilever 115 length may beshorter or longer than the pitch. This is because of the offset in probetips 112 between adjacent rows of probes 100. That is, the distancebetween probe tips 112 on the same plane (the two probe tips 112connected by the line labeled P in FIG. 1, for example) must match thepitch in order to have a probe tip 112 correspond with each solder bump220. However, this does not necessitate any particular cantilever length115 because probe tips 112 on the same plane matching the pitch is notaffected by the length of a cantilever 115. The cantilever 115 lengthmust be sufficient to provide the flexibility to bend back to contactthe substrate 120 and also sufficient to have enough force to move backtoward its initial position to follow the movement of the solder bump220 as needed. Further, the cross section of the cantilever 115 must besufficient to carry the amount of current needed for testing from thevias 130 through the base 110 to the probe tip 112.

FIG. 2 is a cross-sectional view of a wafer test system 200 according toan embodiment of the invention. A perspective side view of the probes100 on a substrate 120 is also shown in FIG. 2 (bottom right). The chipunder test 210 is shown with one solder bump 220 for simplicity, but itshould be understood that a typical chip under test 210 would generallyhave many solder bumps 220 with a given distance between them (pitch)that dictates the distance between the probes 100. The base 110 of eachprobe 100 is coupled to the probe tip 112 via a cantilever 115. Thelength (A) of the cantilever 110, the diameter (B) of the probe tip 112(assuming a cylindrical exemplary probe tip 112), the height (C) of theprobe tip 112, and the height (D) of the base 110 are all customizableand contribute to the predetermined and desired rigidity and complianceof the probe 100. For example, each of the parameters may be differentbased on the corresponding circuit under test 210. The height D of thebase 110 may determine the maximum deflection of the probe 100, which isdriven by the expected thermal movement of the substrate 120 or solderbump 220. That is, the cantilever 115 can flex (as indicated by thearrows) as the circuit under test 210 is lowered to initiate contactbetween the probe tip 112 and the solder bump 220, but the cantilever115 will stop bending when the probe tip 112 contacts the substrate 120(FIG. 1) at a deflection equal to the height D of the base 110. Otherembodiments of the probe 100 in which the maximum deflection of thecantilever 115 is greater than or less than the height of the base 110are discussed with reference to FIGS. 3 and 4 below. As noted above, thelength, thickness, and width of the cantilever 115 must be such that thecantilever 115 has sufficient strength to flex back (down in theorientation shown in FIG. 2) to initiate contact yet flex back up tofollow any movement of the solder bump 220. The length A of thecantilever 115 may be, for example, 25-500 μm. The diameter B of theprobe tip 112 is selected to ensure clearance for adjacent probes 100used in a fine pitch circuit under test 210. As noted above, thediameter B of the probe tip 112 is also based on the material it is madefrom (versus the material of the solder bump 220).

FIG. 3 is a perspective side view of a probe 100 according to anembodiment of the invention. According to the present embodiment, thecantilever 115 may be directly coupled to the substrate 120 rather thanthrough a base 110. Alternatively, the base 110 may be regarded as thatportion of the cantilever (116) connected to the substrate 120. Even ifthe cantilever were coupled to the substrate 120 through a base 110, thecantilever 115 according to the present embodiment angles up (accordingto the orientation shown in FIG. 3) and would not be on the same planeas the top of the base (as shown in FIG. 2, for example). Thus, themaximum deflection of the cantilever 115 would be greater than theheight of the base 110.

FIG. 4 is a perspective side view of a probe 100 according to anembodiment of the invention. In this case, the cantilever 115 is coupledto the substrate 120 through a base 110 but the cantilever 115 is angleddown (according to the orientation shown in FIG. 4) such that themaximum deflection of the cantilever 115 (before the probe tip 112 restson the substrate 120) is less than the height (D) of the base 110.

FIG. 5 illustrates the deflection of the probes 100 according to anembodiment of the invention. As shown, the probe tip 112 and cantilever115 may be pushed down from an initial position (a) to a deflectedposition (a′). The force needed for deflection of the cantilever 115 andprobe tip 112 may be on the order of 0.1-3 grams per probe 100, forexample. As shown in exemplary FIG. 3, the deflection of the probe tip112 is 3 μm. This small mechanical force between the solder bump 220 andprobe tip 112 is insufficient to break thru the oxides and make reliableelectrical contact. After the probe tip 112 contacts the substrate 120,the probe 100 force (force applied by the probe tip 112 up into a solderbump 220) can increase to sufficient levels (10 to 40 gms) to deform thesolder bump 220 and, thereby make electrical contact without causing anydamage to the probe. To be clear, when all the probes 100 are consideredtogether, the wafer probing is initiated in the following way. All ofthe cantilevers 115 of all of the probes 100 flex back (toward thesubstrate 120) until all of the probe tips 112 contact the substrate120. Contact force between every probe tip 112 and corresponding solderbump 220 is increased from this position to ensure that every probe tip112 establishes electrical contact with every corresponding solder bump220. Because all the solder bumps 220 are not uniform, some of thesolder bumps 220 will be deformed more than others during thiselectrical contact initiation.

The probes 100 may be regarded as rigid probes 100 with reversecompliance because the probe tip 112 remains rigid at high forces but isfree to move away from the substrate 120. During testing, based onthermal and/or mechanical stresses, if the solder bump 220 moves closerto the probe tip 112, the deflection of the solder bump 220 increases.If, on the other hand, the thermal and/or mechanical stresses duringtesting cause the solder bump 220 to move away from the probe tip 112,the previously deflected cantilever 115 moves back up and therebyfacilitates continued contact between the probe tip 112 and the solderbump 220. Because of the reverse compliance of the cantilever 115, theprobe tip 112 follows the movement of the solder bump 220 (toward andaway from the probe tip 112) during testing such that the electricalcontact between the probe tip 112 and solder bump 220 is maintainedthroughout the testing. Since the oxide layer on the solder bump 220 hasalready been pierced and electrical contact initiated, the forcenecessary to maintain contact is less, and is provided by the cantilever115.

Again, when the entire array of solder bumps 220 of a given circuitunder test 210 are considered, the importance of rigid probes withreverse compliance becomes even clearer. The array of solder bumps 220of a circuit under test 210 may be non-uniform in size and non-parallelto the substrate 120 (non-planar). After each probe tip 112 contacts thesubstrate 120 and becomes a rigid probe, sufficient additional force isapplied so that all the solder bumps 220 are compressed by therespective (rigid) probe tips 112. Because the solder bumps 220 are notelastic, less than 1 μm of movement can cause loss of contact betweenthe solder bump 220 and corresponding probe tip 112. A large force isrequired to make initial contact to all the solder bumps 220, but only arelatively small force is required to maintain electrical contact. Thus,embodiments of the invention include a cantilever 115 with small forceand small deflection designed into the small spaces between the solderbumps 220. As a result, if every probe tip 112 is not associated with areverse compliant cantilever 115 that facilitates individual probe tips112 following the movement of individual solder bumps 220 independentlyof other probes 100, only some of the probes 100 would maintainelectrical contact with some of the solder bumps 220 during high powertesting. While embodiments described herein discuss the circuit undertest 210 as being lowered onto the probes 100 such that the deflectionof the cantilever 115 is downward, other orientations are alsocontemplated. For example, if the probe tip 112 shown in FIG. 2 wereoriented downward and the solder bump 220 were pushed up to initiatecontact between the probe tip 112 and the solder bump 220, thecantilever 115 would be deflected up (still toward the substrate 120 onwhich the probes 100 are disposed) and may move back down, based on thesubsequent movement of the solder bump 220, to maintain contact.

FIG. 6 is a flow diagram of an exemplary method 400 of probingcompressible bumped or column structures according to an embodiment ofthe invention. As discussed above, the compressible bumped structuresmay be solder bumps 220. Disposing the probe 100 on the substrate 120(block 610) includes arranging adjacent probes 100 at a distance toaccommodate the pitch of the circuit under test 210. At block 620,applying force to the probe 100 with the circuit under test 210 includesaligning each solder bump 220 with a corresponding probe tip 112 of aprobe 100 and pushing against the probe tip 112 with the solder bump 220of the circuit under test 210. The cantilever 115 flexes, in response tothe force applied with the circuit under test 210, until the probe tip112 contacts the substrate 120 (block 630). The method 400 also includescontinuing to apply force to the probe 100 (to all the probes 100 on thesubstrate 120) to ensure that every probe tip 112 has contacted thesubstrate 120 (i.e. every cantilever has bottomed out (block 640). Thatis, when the probes 100 are rigid reverse compliant probes as discussedabove with reference to the various embodiments of the invention, thecantilever 115 of each probe 100 is initially deflected in a directionaway from the solder bump 220 into contact with the substrate 120 onwhich the probe 100 is attached at the base 110 as shown in FIG. 2. Atblock 440, applying additional force to the probes 100 causes the probetip 112 to push into the corresponding solder bump 220. In this way, theprobe tip 112 makes electrical contact with the solder bump 220 (block650). At block 660, applying current and testing the circuit under test210 includes via connections 130 (FIG. 1) supplying current to theprobes 100 (bases 110) through the substrate 120. The cantilever 115carries this current from the base 110 to the probe tip 112 (block 670)to conduct the testing by applying this current to the solder bump 220.When temperature changes result in movement of the solder bump, thecantilever maintains electrical contact between the probe tip 112 andthe associated solder bump 220 (block 680) by flexing back toward thesolder bump 220 as needed.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of onemore other features, integers, steps, operations, element components,and/or groups thereof.

The description of the present invention has been presented for purposesof illustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

The flow diagram depicted herein is just one example. There may be manyvariations to this diagram or the steps (or operations) describedtherein without departing from the spirit of the invention. Forinstance, the steps may be performed in a differing order or steps maybe added, deleted or modified. All of these variations are considered apart of the claimed invention.

While the preferred embodiment to the invention had been described, itwill be understood that those skilled in the art, both now and in thefuture, may make various improvements and enhancements which fall withinthe scope of the claims which follow. These claims should be construedto maintain the proper protection for the invention first described.

What is claimed is:
 1. A method of probing compliant bumps of a circuitwith probes, the method comprising: disposing the probes on a substrate,a base of each of the probes being coupled to the substrate; disposingthe circuit such that each of the compliant bumps is in contact with theprobe tip of a corresponding one of the probes, each probe tip beingconnected to each base of each probe through a cantilever, a thicknessof each cantilever being between 2-5 micrometers (μm), a length of eachcantilever being between 110-150 μm, and a width of each cantileverbeing between 10-20 μm; applying force with the circuit to cause everyone of the cantilevers to flex from a first position to a secondposition at which the corresponding probe tip is stopped from furthermovement by the substrate; continuing to apply force until each of theprobe tips deforms and makes electrical contact with each of therespective compliant bumps; supplying current to the probes to test thecircuit.
 2. The method according to claim 1, wherein the disposing theprobes includes setting a distance between adjacent probe tips of theprobes based on a distance between adjacent compliant bumps.
 3. Themethod according to claim 2, wherein the setting the distance includessetting a distance between the probe and an adjacent probe of 45 to 65micrometers (μm).
 4. The method according to claim 1, wherein thedisposing the probes includes disposing the probes with a shape of eachprobe tip being based on metallurgy of the probe tip and materialcomprising the compliant bump.
 5. The method according to claim 1,wherein the supplying current to the probes is through via connectionsof the substrate.
 6. The method according to claim 1, further comprisingsupplying current from the via connections of the substrate in contactwith each base to the respective probe tip of each probe through therespective cantilever.
 7. The method according to claim 1, furthercomprising maintaining contact between each of the compliant bumps andthe corresponding probe tips based on a compliant characteristic of thecantilever of each of the probes, such that the cantilever moves fromthe second position toward the first position to follow any movement ofthe compliant bump.
 8. The method according to claim 7, whereinmaintaining contact includes the cantilever remaining affixed at a firstend coupled to the base and flexing at a second end coupled to the probetip.